1. Field of the Invention
The present invention relates to a method for fabricating a star coupler, and a fabricated star coupler, for interconnecting a plurality of optical fibers to deliver and to distribute optical signals.
2. Description of the Related Art
In optical communication systems, star couplers are used for delivering and distributing optical signals. For example, a plurality of optical fibers are connected to the star coupler on the input side thereof, and an additionally plurality of optical fibers are connected to the star coupler on the output side thereof. A light propagating in one of the optical fibers on the input side is made incident to the star coupler and spreads into all the optical fibers on the output side to distribute optical signals.
There are star couplers of the waveguide type and star couplers of the fused optical fiber type. The star couplers of the waveguide type are fabricated separately from optical fibers, and must be subsequently connected to optical fibers. The star couplers of the fused optical fiber type are fabricated from optical fibers and can be advantageously and easily connected to other optical fibers with a small insertion loss.
The star coupler of fused optical fiber type is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 1-93706. This star coupler is fabricated by the steps of bundling a plurality of optical fibers, heating the optical fibers to cause the optical fibers to be melted and fused together, and stretching the optical fibers.
The optical fibers usually consist of cores, clads, and coats and, when making the star coupler of the fused optical fiber type, it is necessary to partially remove the coats of the optical fibers to reveal bared fiber portions corresponding to the location of the star coupler. Otherwise, it is possible to use entirely coatless optical fibers for making the star coupler. However, the entirely coatless optical fibers are difficult to handle.
In the fabrication of the conventional star couplers of the fused optical fiber type, it is necessary to hold and handle a group of optical fibers using special tools. However, the use of the special tools requires a more complicated apparatus and a more complicated operation. In the method for fabricating a star coupler described in the above described prior art, a glass tube is used into which a group of optical fibers are inserted. The glass tube is then heated from the outside thereof so that the glass tube shrinks and the optical fibers are molten and fused together. The optical fibers with the glass tube are subsequently stretched. By using the glass tube, it is possible to retain the group of optical fibers in the glass tube. Light is confined within the glass tube, as it has a lower index of refraction, in order to prevent leakage of the light after stretching, and it is thus possible to reduce the insertion loss.
The method for fabricating a star coupler, described in this prior art, is adapted for use with coatless optical fibers, but it is not adapted for use with coated optical fibers. That is, it is difficult to handle optical fibers having no covers along the length thereof, and coated optical fibers must be prepared by partially removing the coats from the optical fibers so that bare fiber portions appear in the intermediate position of the optical fiber corresponding to the region where the star coupler is to be fabricated, with the covers maintained unremoved at the end portions of the optical fibers.
However, in order to insert optical fibers having the uncoated intermediate portions and the coated end portions into the glass tube, it is necessary to first insert the coated end portions of the optical fibers into the glass tube. The diameter of the covered end portion of the optical fiber is larger than the diameter of the uncoated intermediate portion of the optical fiber, so the glass tube must have a relatively large inner diameter to allow the covered end portions of the optical fibers to pass through the glass tube. Therefore, a gap between the glass tube and the uncoated intermediate portions of the optical fibers becomes large, with the result that the optical fibers are apt to move in the glass tube and to deform during the heating step before the glass tube sufficiently shrinks to hold the optical fibers in place. Therefore, the prior art suffers from the problems that optical fibers constituting the star coupler are not uniformly coupled and stretched, and it is not possible to reliably fabricate star couplers having uniform branching losses.